The long-term goal of the proposed research is to characterize the intrarenal regulation of electrolyte transport. It is proposed to study two major hormonal systems, parathyroid hormone and mineralocorticoids. The hypotheses to be tested include: a) parathyroid hormone decreases isotonic reabsorption by the proximal tubule by an effect primarily on anion reabsorption; b) parathyroid hormone inhibits bicarbonate and phosphate reabsorptions in the proximal tubule through inhibition of carbonic anhydrase; c) phosphate is reabsorbed in the papillary collecting duct in the absence of parathyroid hormone; and d) that parathyroid hormone inhibits phosphate transport in this nephron segment. These problems study of proximal tubule function and in hamsters for the study of papillary collecting duct function. Microanalysis will be performed utilizing a newly developed micro CO2 apparatus and electron probe methodology. The role of mineralocorticoids in the regulation of sodium and potassium transport in the papillary collecting duct will be studied. The hypotheses to be tested include: a) the sodium retaining and potassium losing properties of known mineralocorticoid scan be dissociated at the level of the papillary collecting duct and that b) that newly identified mineralocorticoids can be sodium retaining without stimulating potassium secretion in the papillary collecting duct. These problems will be investigated utilizing micropuncture techniques in the hamster and microanalysis by helium glow photometry.